Hi Paul, from the screen capture, you've configured SAML2 custom application with 'Use unique ID' enabled, the error was thrown from ISV when the application has sent an AuthnRequest message to ISV, from screen capture, the application sent AuthnRequest to ISV end point at /saml/sps/saml20ip/saml20/291f9378b4624a68
This end point URL looks not correct, it might be /saml/sps/saml20ip/saml20/291f9378b4624a68/login, can you try to update your twitter application configuration and set the IdP login url with above to include /login suffix?
If you open your application in ISV /ui/admin/application, in the second tab 'Sign-on', scroll the panel to the right, Login URL under text description "If the Use unique ID check box is selected, use the following value:" could be used to configure the SP application.
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Yongming Chen
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